1. Field of the Invention
The present invention relates to integrated circuit manufacturing; in particular, the present invention relates to methods for isolating devices on an integrated circuit.
2. Discussion of the Related Art
Local oxidation of silicon (LOCOS) has been widely used for isolating active components fabricated on the surface of a semiconductor substrate. Typically, LOCOS is used to create a region of electrically insulating silicon oxide, usually referred to as "field oxide", between two regions ("active regions" or "island regions") in which circuit elements or devices are fabricated. Without proper control, LOCOS creates an oxide region which grows not only in the vertical direction but also laterally. Thus, a field oxide formed under LOCOS suffers from two disadvantages. First, the field oxide "encroaches" into the active regions, forming a tapering structure (commonly referred to as a "bird's beak") extending into the active regions. Consequently, active regions have to be given additional area to compensate for this encroachment, such that the device density achievable in the integrated circuit is adversely affected. Second, the field oxide grows vertically, so that the semiconductor surface lacks planarity. Maintaining planarity is essential to successful fabrication of modern complex integrated circuits.
Various approaches are known in the prior art to enhance planarity. In one technique, known as recessed isolation, a groove or trench of predetermined depth is cut by an anisotropic etch into the semiconductor substrate. Under that approach, the field oxide is grown by LOCOS from the bottom of the groove or trench. Since it is grown from a recessed position, the resulting field oxide does not disrupt surface planarity as its non-recessed counterpart. However, because this approach requires an etching of silicon to create the trench or groove, undesirable damage to the semiconductor surface may result.
Various approaches have also been attempted in the prior art to limit the field oxide's encroachment into an active region. In one such technique, an oxidation resistant material, such as nitride or certain polymers, is provided on the side walls of the groove or trench, so as to check the lateral growth of the field oxide. Such techniques tend to require a high degree of process control to deliver the intended result.